Be patient, you'll get a message of successful upload and the router will reboot, it will take a while, led will always be orange from now on so don't worry, as soon as it pings you can login on DD-WRT web interface.

The firmware language defaults to chinese, on the usual first boot screen set username, password, password confirmation.

Then follow this to set the language to English:

Scroll down and select English from the dropdown box:

Scroll at the end of the page and click Apply Settings:

Then go to Services->NAS and enable Samba, reboot the router and it will automount the internal HD.
All the files you had on it will be in /mnt/sda4

If you wish to repartition your HD, easiest way is to take it out, connect it to a PC (USB or internal) and partition it (ext2/ext3) with an utility (Acronis Disk Director for Windows or GParted for Linux).

FAN Noise

If you think the FAN is too loud, you can turn off fan speed management and it will run at the slowest speed.
Open an SSH connection and use this commands:

nvram set fancontrol=off
nvram commit

Going back to the original firmware

1. Download miwifi-stock.bin and copy it on the internal HD of the router.